What is the maximum current in a superconductor loop?

[fxdung]

If we put an emf equal constant onto a superconductor loop(we consider the resister of emf equal zero), then what is the maximum of current in the loop?

 

[mfb]

Your superconductor will stop being a superconductor at a current that depends on the temperature and external magnetic field strength, and sometimes the current direction (and the material obviously).

 

[Gunnir]

To just elaborate on mfb's answer a bit, we typically define the maximum limiting current (density) in a superconductor by what is called the 'critical current': J_c.

Superconductors come in a variety of flavours, and the 'critical current' (density) is intrinsic to each superconductor material, fabrication process, shape, quality, and many more factors! Ultimately however, in type-2 superconductors which heed all real practical applications, the current that you can pass through the superconductor depends on the pinning centres of the superconductor. Pinning centres are regions of non-superconducting states; which sounds counterintuitive at first, but actually introducing non-superconducting regions helps `pin' and retain superconductivity. These pinning centres are thermally and magnetically dependent; depend on material, sample shape, crystalline quality, growth sectors, manufacture technique, size and relative number of impurities/defects - so to reiterate, are many, many factors!